Ideally the above lens filters are to be the preferred type so the light path via the lens is not compromised, however in practise it will make very little or no difference. The Ilford below lens filter outfit comes with a 'cage' (The best way of describing it) which has an open end with no plastic and the other end is circular and will fit between the 39mm lens screw thread and the lens mount.

Another reason why I used to prefer below the lens filters is the ease of changing filters if I was doing a print which required a number of different grades. If you have the type that fit in a filter drawer then there is a chance that when changing the filter you may just knock the enlarging head out of alignment.

As I use a colour head for all my prints I have never experienced this problem.